In current imaging systems such as a digital still camera or a video camera, the use of a single-plate imaging device having a complementary color filter or a primary color filter on a front surface has become mainstream. A color image signal from the imaging device is subjected to white balance processing and interpolation processing, then subjected to edge enhancement processing, chroma enhancement processing, tone conversion processing, and so on, and then output. This processing may be performed directly on the color image signal from the imaging device, but normally the processing is performed after converting the color image signal into a separate color space to divide it into a luminance signal and a color signal. In this case, the color signal may become unnatural when only the luminance signal is modified through tone conversion processing and so on.
In response to the problem of unnaturalness in the color signal, JP2004-312467A discloses a method of correcting the chroma of the color signal when the luminance signal is modified such that a ratio thereof relative to a maximum chroma value of the color space becomes fixed.
Further, JP3-127278A discloses a method of achieving natural imaging in computer graphics by determining a ratio between an original luminance signal and a corrected luminance signal, and using the result as a correction coefficient that is multiplied by a color signal.
Further, JP2003-69846A discloses a method of correcting color cast by dividing a color space into a plurality of hue regions and determining the hue region to which an image signal belongs in pixel units. A pixel having a high possibility of color cast is then determined by multiplying the pixel by a weighting factor from 0 to 1 on the basis of a different function for each hue region, whereupon a color cast correction coefficient is determined from the determined pixel.